1. Field of the Invention
The present invention relates generally to optical communication systems and, particularly to Coarse Wavelength Division Multiplexed (CWDM) fiber optic communication systems with small size optical transceivers.
2. Description of Related Art
Wavelength Division Multiplexing (WDM) technologies have been widely used in optical communication systems to provide much greater bandwidth than single wavelength communication systems. The wavelength spacing between channels in CWDM systems are much larger than that in DWDM systems, which allows the use of uncooled lasers. CWDM optical communication systems typically utilize 4 to 16 channels with about 20˜25 nm channel spacing to provide low cost solutions to increase the bandwidth of a single fiber link. For example, IEEE 802.3ae 10 Gb/s Ethernet standard specifies a CWDM link so called LX4. The LX4 standard specifies four channels with channel wavelength centered at 1275.7 nm, 1300.2 nm, 1324.7 nm and 1349.2 nm. The wavelength range of each channel is 13.4 nm to accommodate wavelength drift of the laser over temperature. Each channel provides a 2.5 Gb/s data bandwidth. Other CWDM systems covers wavelength from 1610 nm, 1590 nm, 1570 nm, down to 1310 nm, and 1290 nm with 4˜16 channels at a 20 nm channel spacing. In addition, there are other CWDM systems using vertical cavity surface emitting lasers (VCSEL) in 800 nm range, at wavelength from 780 nm to 860 nm. In CWDM transceiver modules, there is a need to multiplex the light from multiple lasers at different wavelengths to couple into a single fiber for light transmission. The CWDM transceivers typically need to support single mode, multimode, or both types of optical fiber.
Typically, for such a CWDM transmission system, the light from the laser at each wavelength is first coupled into an optical fiber using a fiber pigtailed package or a receptacle package. Then the light from multiple fibers is multiplexed into one fiber using an external fiber based multiplexer outside of transmission laser package. The fiber based multiplexer has multiple input fibers and one output fiber. FIG. 1 shows an example of such optical systems with four channels. Each of four lasers is individually packaged in a fiber pigtailed package 104a, 104b, 104c and 104d. Each single package is a single channel transmitter optical sub assembly (TOSA). For example, the pigtailed package 104a includes a laser die 101a, a coupling lens 102a, an isolator 103a, and a fiber pigtail 105a. Four fiber pigtails, 105a, 105b, 105c, and 105d are combined together through a fused fiber couple, which functions as a multiplexer with a fixed 6 dB loss in each channel. Generally, a fiber based multiplexer can be a fused fiber coupler with high loss, or a thin-film filter based zigzag optical path design with low loss, or other design such as using a gratings. Four individual TOSA and the external multiplexer together constitute a CWDM TOSA. This type of CWDM TOSA is bulky, costly, and involves significant challenges to manage the fiber inside a transceiver module. For example, to package four pigtailed lasers and the fiber based multiplexer into a LX4 transceiver module with limited space is very challenging and costly. Therefore, this approach is limited in the applications with large form factors, such as a XENPAK transceiver, not suitable for the application that uses small size optical transceiver modules, such as a X2, XFP or smaller form factors.
An alternative approach is to integrate the CWDM lasers and the multiplexer into a single package. The typical multiplexer for integration with lasers is a thin-film filter (TFF) based zigzag optical path design. For example, U.S. Pat. No. 6,769,816 by Capewell et al, and U.S. Pat. No. 6,201,908 by Grann describe transmitter systems with such an approach. The main drawback of this approach is that the optical path lengths are long and different for each channel. The very long optical paths for some channels often impose very tight package assembly and alignment tolerance. They also require large optical components such as TFF if standard transistor outline (TO) package lasers are used. Other multiplexer technologies can be integrated with lasers, such as grating-based multiplexer, planar light circuit, etc. In general, to keep the size small and optical paths relative short, lasers in bare die form are usually necessary to use these multiplexing approaches. They typically require expensive alignment and assembly equipments, as well as expensive processes to make such a CWDM TOSA. The size and cost, as well as performance over temperature and the long-term stability are still concerns to use a CWDM TOSA based on these designs. Thus such approaches are typically applicable to multimode applications with loose tolerances. They are very difficult to implement in single mode fiber applications, such as that required in LX4 standard.
Therefore, there is currently a demand for compact, cost effective, and highly integrated CWDM TOSA for optical communication systems, particularly inside optical transceivers. The present invention discloses a multi-wavelength TOSA with integration of lasers and a multiplexer in one package. The invention allows using either traditional low cost and proven TO-Can packaged lasers or lasers in bare die forms. The optical paths of all channels are short and possibly same length for all channels to have better tolerances.